Method for reclaiming the selvage of foamed thermoplastic web

ABSTRACT

A method of reclaiming the selvage remaining in an extruded sheet of foamed thermoplastic material after the molding of articles from the sheet, in which the reclaiming steps are integrated with the extrusion and forming steps in a continuous system. Immediately after forming the articles and cutting them from the sheet, the foamed selvage is collapsed and densified before air has penetrated the cells thereof, and thereafter it is granulated, mixed with virgin extrusion pellets, and the mixture is returned to the extrusion step for re-use.

United States Patent 91 Winstead 1 Mar. 27, 1973 METHOD FOR RECLAIMINGTHE 2,744,291 5/1956 Stasmy et al. ..264/DIG. 4 SELVAGE 0 FOAMED3,082,483 3/1963 Bickford ..264/DIG. 4 THERMOPLASTIQ WEB 3,607,9999/1971 Corbett et al.; ..264/5l X [76] Inventor: Thomas W. Winstead, 2Overlook OTHER PUBLICATIONS z Baltimore Collins, F. H. ControlledDensity Polystyrene Foam [22] Filed: May 22, 1970 Extrusion," ln SPEJournal, July 1960, pp. 705-709.

[21] Appl' 39.851 Primary ExaminerPhilip E. Anderson Related US.Application Data 0 neyRap ael semmes [63] Continuation-impart of Ser.No. 719,057, April 5, is

1968, abandoned. [57] TRACT A method of reclaiming the selvage remainingin an [52] US. Cl. ..264/37, 264/53, 264/ 141, extruded sheet of foamedthermoplastic material after 264/321, 264/DIG. 69 the molding ofarticles from the sheet, in which the [58] Field Of Search 4/ 2 6- 37,reclaiming steps are integrated with the extrusion and 6 forming stepsin a continuous system. Immediately after forming the articles andcutting them from the References Cited sheet, the foamed selvage iscollapsed and densified before air has penetrated the cells thereof, andUNITED. STATES PATENTS thereafter it is granulated, mixed with virginextrusion 3,238,565 3/1966 Jacobs ..264/321 X pellets, and the mixtureis returned to the extrusion 3,189,399 6/1965 Jacobs et a1. step forre-use. 3,475,526 10/1969 Seto 3,531,562 9/1970 Serrano et a1. ..264/32lX 13 Claims, 1 Drawing Figure SELV AGE 9 PRESSURE ROLLER 0 o EXTRUDE DWEB EXTRUDER Z STRIPPINGROLLER 4 FORMED wee 7 5 6 9 FORMED ARTICLES 5CUTTING ROLLER I, GRANULATOR 7 FORMING UNIT l I COLLECTOR PATENTEUHARZY197s ATTORNEY METHOD FOR RECLAIMING THE SELVAGE OF FOAMED THERMOPLASTICWEB REFERENCE TO COPENDING APPLICATION This application is acontinuation-in-part of application Ser. No. 719,057, filed Apr. 5, 1968and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a method forreclaiming and beneficially utilizing the selvage or trim remaining inan extruded sheet of foamed thermoplastic material after the vacuum orblow molding of foamed thermoplastic articles, such as trays, plates,cups, etc.

In the manufacture of low cost packaging items, there is an increasingtrend toward the use of foamed thermoplastics, and the utilization oflow density thermoplastics (of the order of 3 lbs/cu.ft. or'less) forsuch products has resulted in cost performance characteristicscompetitive with low cost conventional materials such as paper. However,since there is a considerable amount of waste through the selvage ortrim resulting from most high speed processes, the efficient re-use ofsuch trim or waste is of particular importance.

In the manufacture of packaging items such as food and produce trays,for example, the trim ordinarily results from the severing of the formedproduct from a continuous, foamed thermoplastic web. One example of asystem of this nature is disclosed in my copending application Ser. No.480,9l7, filed Aug. 19, 1965, now US. Pat. No. 3,479,694, issued Nov.25, 1969, covering a high-speed, continuous system in which the steps ofextruding the foamed sheet, forming, and cutting follow in continuous,rapid succession. The trim is immediately granulated and continuouslyfed back into the extruder along with added virgin material. Dependingupon the shape of the particular article being manufactured, the ratioof granulated trim to virgin material will vary. In most instances,however, the ratio of granulated trim to virgin material ranges from 25percent to 50 percent of the total. At these high ratios, when there-used low density trim is in an expanded state, serious feedingproblems can be experienced in the extruder screw, which can beparticularly detrimental to a foaming process which depends uponstability within the extruder system for efficient output and goodquality.

BRIEF DESCRIPTION OF THE INVENTIONv The primary object of the presentinvention is to provide a method for re-using the trim which normallyresults from the manufacture of foamed articles of low density,including the steps of densifying the trim by crushing it between smoothrollers without addition of heat and before the blowing agent in thecells of the foam has been replaced by air, and then granulating thedensitied trim to a relatively small particle size, so that theresultant material is of sufficiently high density to feed and processefficiently in a single screw extruder system.

Another object is to provide a method of this nature which is extremelysimple and results in improved efficiencies within the extrusion system.

A further object is to provide a system which facilitates themanufacture of end products of higher quality and more uniform cell sizethan when using non-densified foamed trim.

Still another object is to provide a method of reclaiming foamed trim bya simple technique which makes possible the re-use of 50 percent or moreof the trim.

BRIEF DESCRIPTION OF THE DRAWING The invention will be further describedin conjunction with the accompanying drawing, the single FIGURE of whichis a diagrammatic illustration of the invention shown as incorporated inan integrated system for the manufacture of formed articles from foamedor cellular thermoplastics.

DETAILED DESCRIPTION OF THE INVENTION While the invention may beadvantageously used, under appropriate conditions, in a two-stage systemwherein the thermoplastic material is extruded and then reheated forthermoforming in a second operation, the greatest benefit from theinvention can be obtained if it is incorporated in a single stage systembecause of the economy effected by the simplicity of the invention insuch a system, and because of the ease of densifying the trim, as willlater be described. For this reason, the invention is illustrated asintegrated in a single stage system, which may, for example, be of thetype disclosed in detail in my said copending application Ser. No.480,917 and now US. Pat. No. 3,479,694. The invention is not, however,limited to the details of the extruding, forming, and cutting conceptsdisclosed in that application, nor is the invention limited to aparticular thermoplastic material. The invention may be employed, forexample, in reclaiming and re-using trim of foamed polystyrene,polyvinyl chloride, polyvinyl acetate, polyvinyl dichloride, celluloseacetate, cellulose acetate butyrate, vinylidene chloride,.methylmethacrylate, and ABS (acrylonitrile, butadiene and styrene), all ofwhich are well known and conventional in the art of formingthermoplastics into articles such as cups or trays. Polystyrene, forexample, is readily available in virgin pellet form, of cubi cal,cylindrical or other shapes, in sizes such as one-sixteenth toone-eighth inch, and with bulk density of the order of 35 to 40 lbs.percubic foot. Suitable resin stock is available from Foster Grant Co.,Inc. of New York, N. Y., Dow Chemical Co., and Shell Chemical Co.

In the diagrammatic drawing, 1 represents a conventional extruder whichextrudes a foamed thermoplastic web 2 and-feeds it directly to a formingunit :3, which may be either a blow molding or vacuum molding device, asset forth in my aforesaid copending application. The formed web 4carrying a continuous succession of formed articles, such as trays 5, isfed from the forming unit between the cutting roller 6 and the strippingroller 7 where the formed articles are cut and stripped from the web anddeposited in any suitable collector 8, all as described in thatapplication. The remaining trim or selvage 4a passes over the strippingroller 7 between the latter and a pressure-loaded roller 9, from whenceit is passed over rollers 10 and fed to a conventional granulator 11,such as a rotating blade type. The granulated material is deliveredthrough a conduit 12 to a mixer 13 where virgin extrusion pellets areadded, and thence through a conduit 14 for return to the extruder-l,thus completing the cycle. Merely for purposes of example, it may beassumed that general purpose polystyrene is extruded from a diemaintained at 300F. in a 6 inch width at a feed rate of I28 ft./min.,

with a final density of 2.5 lbs./cu.ft., and a final thickness of 0. l25 inch, and that the temperature of the extrudate reaching the formingstation is 230F. The elapsed time between emergence of material from theforming unit 3 and crushing of the scrap. thereof is between and 30seconds.

In conventional methods of manufacturing foamed trays or the like,wherein a two-stage method is em-' ployed, sheeting is first extruded intubular form and collected in'rolls after being slit longitudinally. Thesheeting is then reheated for thermoforming in the secondoperation,which frequently takes place after 24 hours or more haveelapsed, and during this period of storage of the rolled foamedsheeting, the blowing agent permeates from the cell walls and isreplaced by air. However, in a single stage system, such as abovereferred to and described in my said copending application Ser. No.480,917 and now US. Pat. No. 3,479,694, the blowing agent is stillpresent within the cell walls at the time thetrim" is granulated,because very little time has elapsed since the forming operation.Frequently, volatile liquid blowing agents are used which have boilingpoints between 70F. and 100F. Conventional blowing agents of this typeinclude pentane and trichlorofluoromethane. If the trim is cooled belowthe boiling point of the blowing agent and densifying is carried outbefore air permeates the cell walls, a negative pressure is developedwithin the cells, making densification a very simple matter. If, undersuch conditions, the trim is passed between pinch rollers or crushingrollers, it is easily collapsed and densitied into a thinner continuousflat web without the addition of heat and without other complexitieswhich characterize prior systems, andthe resulting density aftergranulation is considerably higher than otherwise. Thisdensification canbe effected either between rollers 7 and 9 or between rollers 10 priorto feeding the selvage 4a to the granulator 11. If, however, a periodof-a half hour or more elapsed after forming and before crushing of thescrap, sufficient air would permeate the cells to make densificationsignificantly more difficult.

In addition to this compression treatment of the trim, a furtherincrease in density may be attained by using a relatively fine screen inthe granulator 11 to produce small particle sizes. For example,frequently in sheet trim granulation, a screen is used with holes ofthreesixteenths inch to three-eighths inch, and I have found that usinga screen between one-sixteenth inch and three-sixteenths inch produces asignificantly higher density.

Theamount of pressure required and the magnitude of other variables isrelative. Typically, rollers of approximately 3 inches to 4 inches indiameter are used for densifying the trim, and pressure in the order of20 to 50 lbs. per linear inch have been found adequate. Density. of thegranulated trim can easily be doubled over that possible without usingthe technique of the present invention. For example, with a low density,foamed trim'having a density of 3 lbs. per cubic foot when granulated inthe conventional manner, the resulting material might have abulk densityof 2 to 5 lbs. per cubic foot, depending upon screen size. However, byusing the method of the present invention, wherein the trim is firstdensified by the use of pressure rollers, and then passed through a 3/32inch diameter screen, the bulk density can be increased to from 6 to 15lbs. per cubic foot. In the case of polystyrene, the bulk density of theraw extrusion compound is in the order of 30 to 35 lbs. per cubic foot.

By using a relatively small screen in the granulator 11, not only is thebulk density of the granulated trim increased, but also the physicalsize of the individual particles is such that they appear to fill thenatural voids between virgin extrusion pellets and, at least-to someextent, thereby further optimize the overall bulk density of themixture.

The simple technique of this invention permits the continuous re-use ofup to 50 percent or more of the trim for admixture with virgin material,without requiring heating, cooling, or re-extrusion of the trim in thereclaiming process, without requiring blending and lubricants, andwithout interruption. Hence, one can appreciate the advantage of thistechniquewhen considering the combined bulk density of a 50-50 mixture(by weight) of granulated trim and virgin pellets, a far moreadvantageous mixture than has heretofore been possible. 1

While the invention has been described as particularlyv adaptable for asingle-stage system, as before stated, it is possible to gain somebenefits if used in a two-stage system. For example, in some instances,a two-stage system reheats and forms immediately or shortly after theextrusion operation, and, in these cases, the effectiveness of thedensifyingmethod of this invention may well be utilized. One of theprimary advantages of the invention resides in the fact that it permitsthe immediate re-use of the trim and retains the residual blowing agentwhich represents a-considerable reduction cost. a

From the foregoing, it is believed that the invention may be readilyunderstood by those skilled in the art without further description, itbeing home in mind that numerous changes-may. be made in the detailsdisclosed without departing from the spirit of the invention.

I claim:

1. In a method of manufacturing articles from foamed thermoplasticpolymeric resin sheet with a density no more than substantially 3lbs./cu.ft., wherein melted foamable thermoplastic polymeric resin stockis mixed with a volatile liquid blowing agent and is extruded as a sheetat a temperature above the boiling point of the blowing agent andwherein the articles are molded sequentially from the sheet and cuttherefrom to leave a selvage web, the improvement which comprisescollapsing and densifying the selvage web by the application of pressurethereto, without additional cutting steps, said selvage web being fedcontinuously and being collapsed and densified and granulated while fedcontinuously.

3. In a method as claimed in claim 2, the granulating of the selvage webcomprising cutting the selvage web into particles and screening saidparticles.

4. In a method as claimed in claim 3, the particles being screened to aparticle size in the range from substantially one-sixteenth inch tosubstantially three-sixteenths inch.

5. In a method as claimed in claim 3, the screened particles filling thevoids between pellets and providing a more continuous mass forextrusion.

6. In a method as claimed in claim 2, the thermoplastic polymeric resinstock being at least 25 to 50 percent granulated selvage web by weight.

7. In a method as claimed in claim 2, said selvage web being collapsedand densified and granulated before blowing agent remaining in the cellsthereof has escaped, whereby the retained blowing agent in thegranulated selvage web is re-used.

8. In a method as claimed in claim 2, said selvage web being collapsedsubstantially uniformly to produce a substantially flat sheet forgranulating.

9. In a method as claimed in claim 2, the blowing agent having a boilingpoint in the range from substantially 70F. to substantially 100F. a

10. In a method as claimed claim 2, the selvage web being densified to adensity within the range of from substantially 6 lbs./cu.ft. tosubstantially l5 lbs./cu.ft.

11. In a method as claimed in claim 2, the selvage web being collapsedand densified and granulated without the addition of other materialsthereto. 7

12. A method in accordance with claim 1, wherein the polymeric resinstock is selected from the group consisting of polystyrene, polyvinylchloride, polyvinyl acetate, polyvinyl dichloride, cellulose acetate,cellulose acetate butyrate, vinylidene chloride, methyl methacrylate,and acrylonitrile butadiene styrene.

13. A method in accordance with claim 1, wherein the collapsing and'densifying is performed of the order of 5 to seconds after the molding.

2. In a method as claimed in claim 1, wherein the sheet is continuouslyfed during the extruding and cutting steps, said selvage web being fedcontinuously and being collapsed and densified and granulated while fedcontinuously.
 3. In a method as claimed in claim 2, the granulating ofthe selvage web comprising cutting the selvage web into particles andscreening said particles.
 4. In a method as claimed in claim 3, theparticles being screened to a particle size in the range fromsubstantially one-sixteenth inch to substantially three-sixteenths inch.5. In a method as claimed in claim 3, the screened particles filling thevoids between pellets and providing a more continuous mass forextrusion.
 6. In a method as claimed in claim 2, the thermoplasticpolymeric resin stock being at least 25 to 50 percent granulated selvageweb by weight.
 7. In a method as claimed in claim 2, said selvage webbeing collapsed and densified and granulated before blowing agentremaining in the cells thereof has escaped, whereby the retained blowingagent in the granulated selvage web is re-used.
 8. In a method asclaimed in claim 2, said selvage web being collapsed substantiallyuniformly to produce a substantially flat sheet for granulating.
 9. In amethod as claimed in claim 2, the blowing agent having a boiling pointin the range from subStantially 70*F. to substantially 100*F.
 10. In amethod as claimed in claim 2, the selvage web being densified to adensity within the range of from substantially 6 lbs./cu.ft. tosubstantially 15 lbs./cu.ft.
 11. In a method as claimed in claim 2, theselvage web being collapsed and densified and granulated without theaddition of other materials thereto.
 12. A method in accordance withclaim 1, wherein the polymeric resin stock is selected from the groupconsisting of polystyrene, polyvinyl chloride, polyvinyl acetate,polyvinyl dichloride, cellulose acetate, cellulose acetate butyrate,vinylidene chloride, methyl methacrylate, and acrylonitrile butadienestyrene.
 13. A method in accordance with claim 1, wherein the collapsingand densifying is performed of the order of 5 to 30 seconds after themolding.